Conventional inline variable displacement piston pumps of the subject type comprise a case or housing within which a cylinder block is coupled to a rotatable drive shaft. The cylinder block contains a plurality of cylinder cavities disposed in a circumferential array surrounding the shaft axis. A corresponding plurality of pistons are slidably positioned within the respective cylinders. The pistons engage a yoke cam which is variably positionable within the pump housing for collectively adjusting stroke or displacement of the pistons within the cylinders. The cylinder block rotates against a valve plate having arcuate inlet and outlet kidney-shaped slots which serve in a well-known manner to provide properly phased or timed communication between the end ports of the cylinder bores within which the pistons reciprocate and inlet and outlet passages and ports in the pump housing.
Timing of the hydraulic pump by circumferential positioning of the slot ends in the valve plate involves matching pump cylinder pressures to inlet and outlet passage pressures at the angular position at which the cylinder begins to communicate through the slot with the inlet and outlet ports. Thus, pump timing is conventionally optimized for only one set of operating conditions--i.e., one design combination of inlet and outlet pressures, pump speed, fluid flow, fluid temperature and fluid type. Deviation from these optimum or design conditions creates under compression or over compression of fluid in the cylinder block, causing high fluid velocities at edges of the timing slots, noise, fluid cavitation, pump wear and flow oscillations resulting in pressure ripple. All of these effects are undesirable in controlled hydraulic circuits.
It has been normal practice to operate a pump at constant pressure conditions by varying pump displacement. However, microprocessor-based control systems provide facility for enhanced control in a plurality of otherwise desirable pump operating modes, such as constant flow and constant power modes. However, pump timing is not optimum for conditions which depart from the pump design conditions, resulting in the various problems noted above.
A general object of the present invention is to provide a rotary hydraulic machine, such as an inline variable displacement piston pump, in which pump port timing varies with operating conditions. A more specific object of the invention is to provide a machine of the described character in which timing is optimized for two sets of operating conditions, specifically high and low output pressure conditions. Thus, a yet more specific object of the invention is to provide dual pressure timing for axial-piston rotary hydraulic machines such as variable displacement piston pumps.